One goal of research on intact auditory systems is to provide a basis for studying the deficits produced by hearing loss and for understanding how damage to the ear may alter the way the brain normally codes information from communication sounds such as speech. A central aim of the proposed study is to evaluate neural activity of central auditory neurons in frogs under conditions that simulate sudden hearing loss, a deficit that produces an acute, rapid-onset hearing impairment. Sudden hearing loss often occurs in one ear, can show partial recovery, and may recur in repeated bouts resulting in a progressive and permanent hearing loss. More than forty causes have been associated with this deficit. The proposed study is unique in that it will evaluate neural function during a rapidly induced hearing loss in non-human animals without damaging the ear. This approach allows the direct comparison of neurophysiological activity under conditions of intact hearing with the effects of hearing loss in each central auditory neuron. The reversible hearing loss is induced by cooling of an auditory nerve where it passes from the ear to the brain to create a temporary hearing loss from one ear. Local cooling of the nerve is achieved by placing the tip of a small cooling probe (< 0.15 mm) close to the nerve; the cooling stops transmission of activity in the nerve, the effect is rapidly reversible upon passive warming, and the procedure is repeatable. The effect of hearing loss will be investigated in auditory nerve cells that typically receive information from both ears. The hypothesis is that hearing sensitivity and frequency selectivity will be poorer with unilateral hearing loss than with intact binaural hearing; responses to monaural stimulation but with intact hearing are expected to show intermediate characteristics. Neurophysiological recordings of single unit activity will be made to evaluate binaural responses, characteristic frequency, threshold, rate-level functions, and frequency response areas. Neural responses will be evaluated when the temporary hearing loss is induced on the same and the opposite side of the recording site in the auditory midbrain to define the contribution of each ear. The focus of this study is to understand how hearing loss in one ear affects the sensitivity and selectivity of auditory neurons for encoding sound.